In preparing an internal gear and an external gear, the external gear rotates with respect to the internal gear when the external gear is caused to revolve orbitally around a central axis of the internal gear while the two are maintaining a meshed state. An eccentrically oscillating type of reduction gear transmission that utilizes this phenomenon has been developed, and such is taught, for example, in Japanese Patent Application Publication No. 1997-57678. This type of reduction gear transmission comprises an internal gear, an external gear, a carrier, a crankshaft, and a bearing.
The external gear rotates while revolving orbitally around the central axis of the internal gear while maintaining a meshed state with the internal gear. A plurality of through holes formed at positions offset from the center are formed in the external gear. The carrier is supported so as to be capable of rotating around the central axis of the internal gear. The crankshaft is attached to the carrier in a manner that allows the rotation of the crankshaft. The crankshaft comprises a shaft part and an eccentric member that has a central axis formed in a position offset from a central axis of the shaft part. The eccentric member of the crankshaft engages with the through hole of the external gear. The bearing is disposed between the carrier and the shaft part of the crankshaft, and supports the shaft part of the crankshaft such that it can rotate with respect to the carrier.
In the configuration described above, when the crankshaft is rotated by a motor, the eccentric member of the crankshaft rotates eccentrically. When the eccentric member of the crankshaft rotates eccentrically, the external gear rotates while revolving orbitally around the central axis of the internal gear. When the external gear rotates, the carrier rotates with respect to the internal gear. The rotation of the crankshaft can be transmitted to the rotation of the carrier, and the number of rotations of the carrier can be reduced compared to the number of rotations of the crankshaft.
In the eccentrically oscillating type reduction gear transmission, the crankshaft must be provided such that it can rotate with respect to the carrier. For example, in the reduction gear transmission of Japanese Patent Application Publication No. 1997-57678, the shaft part of the crankshaft is supported by a tapered roller bearing, and the crankshaft is thus supported such that it can rotate with respect to the carrier.